Conventional network-based communication systems include systems configured to operate in accordance with well-known synchronous transport standards, such as the synchronous optical network (SONET) and synchronous digital hierarchy (SDH) standards.
The SONET standard was developed by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), and is described in the document ANSI T1.105-1988, entitled “American National Standard for Telecommunications—Digital Hierarchy Optical Interface Rates and Formats Specification” (September 1988), which is incorporated by reference herein. SDH is a corresponding standard developed by the International Telecommunication Union (ITU), set forth in ITU standards documents G.707 and G.708, which are incorporated by reference herein.
The basic unit of transmission in the SONET standard is referred to as a synchronous transport signal level-1 (STS-1). It has a serial transmission rate of 51.84 Megabits per second (Mbps).
Synchronous transport signals at higher levels may be concatenated or channelized. For example, an intermediate unit of transmission in the SONET standard is referred to as synchronous transport signal level-3, concatenated (STS-3c). It has a serial transmission rate of 155.52 Mbps. The corresponding unit in the SDH standard is referred to as STM-1. In a concatenated synchronous transport signal, the entire payload is available as a single channel. A channelized signal, by way of contrast, is divided into multiple channels each having a fixed rate. For example, the channelized counterpart to the concatenated STS-3c signal is denoted STS-3. STS-3 is a channelized signal that comprises three separate STS-1 signals each at 51.84 Mbps.
A given STS-3c or STM-1 signal is organized in frames having a duration of 125 microseconds, each of which may be viewed as comprising nine rows by 270 columns of bytes, for a total frame capacity of 2,430 bytes per frame. The first nine bytes of each row are overhead, while the remaining 261 bytes of each row are payload. The overhead includes transport overhead (TOH) and path overhead (POH). Additional details regarding signal and frame formats, such as J0, J1 and other signals, and synchronous payload envelope (SPE) formats, can be found in the above-cited documents.
A drawback of conventional SONET or SDH network-based communication systems relates to the mapping of synchronous transport signals like STS-3c or STM-1 to or from corresponding higher-rate optical signals such as a SONET OC-12 signal or an SDH STM-4 signal. An OC-12 optical signal carries four STS-3c signals, and thus has a rate of 622.08 Mbps. The SDH counterpart to the OC-12 signal is the STM-4 signal, which carries four STM-1 signals, and thus also has a rate of 622.08 Mbps. The mapping of these and other synchronous transport signals to or from higher-rate optical signals generally occurs in a physical layer device commonly referred to as a mapper, which may be used implementing implement an add-drop multiplexer (ADM) or other node of a SONET or SDH communication system.
Such a mapper typically interacts with a link layer processor. A link layer processor is one example of what is more generally referred to herein as a link layer device, where the term “link layer” generally denotes a switching function layer. Another example of a link layer device is a field programmable gate array (FPGA). These and other link layer devices can be used to implement processing associated with various packet-based protocols, such as Internet Protocol (IP) and Asynchronous Transfer Mode (ATM), as well as other protocols, such as Fiber Distributed Data Interface (FDDI).
In mapping synchronous transport signals like STS-3c or STM-1 to or from higher-rate optical signals, a typical conventional mapper is unable to extract and transfer just the payload of the STS-3c or STM-1 signal to the link layer device. This is disadvantageous in that it requires the link layer device to include circuitry for performing the payload extraction operation, which increases the complexity and cost of the link layer device. Also, the link layer device is usually not optimized for such operations, and thus throughput performance may be negatively impacted. Similarly, the typical conventional mapper is unable to receive just the payload of the STS-3c or STM-1 signal from the link layer device, but instead must receive the entire signal including all framing overhead information. Again, this results in additional hardware requirements in the link layer device, in that circuitry is required to add the framing overhead to the payload to form the complete STS-3c or STM-1 signal. Implementing this type of payload insertion operation in the link layer device may also have a negative impact on throughput performance.
Accordingly, a need exists for an improved mapper that is capable of processing synchronous transport signals such as STS-3c or STM-1 without requiring that payload extraction or insertion operations for such signals be performed by a link layer device.